Significant quantities of unburned hydrocarbons exhausted from an internal combustion engine originate within the volume defined by the piston side wall, the upper piston ring and the engine block cylinder wall. The volume, referred to as the cylinder wall crevice volume is of concern in the development of engines with reduced engine-out emissions. Crevice volume reduction can be achieved by reducing the excess clearance between the piston top land and the cylinder bore wall. Numerous factors govern the design of the cylinder wall crevice volume including piston and cylinder bore thermal growth and mechanical distortion, production variabilities, engine dynamics and noise concerns.
Historically pistons have been designed with excess clearance between the piston land and the cylinder wall to avoid excessive wear caused by piston-cylinder contact and to minimize generated noise. Such techniques employed to size pistons have resulted in larger than optimum crevice volumes. Attempts to improve piston sizing have focused on a method of coating the exterior of the piston with an abradable material that wears during engine operation to reveal an improved peripheral configuration. One shortcoming of this method lies in the insulative effect of the coating which leads to a disruption in the heat transfer characteristics within the cylinder. Typically, with this method, piston temperatures are higher than would normally be experienced during operation resulting in increased piston thermal growth and less than optimum final piston dimensions.